Pinocytotic or plasmalemmal vesicles constitute cellular uptake mechanisms which in endothelial and alveolar epithelial cells also serve as pressure-independent shuttle systems for transcellular transport of macromolecules. This study is aimed at gathering histological, morphometric information on the amount and distribution of vesicles in the different cells of the lung by using an on-line computer to process point-counting data obtained from electron micrographs of lung tissue fixed under well defined conditions. The most significant parameter to be computed is the vesicular load or number of vesicles per unit of surface area of each cell front. During the initial period of the grant, morphometric data were obtained on normal and edematous lungs; our current goal is to study the effect of vasoactive substances known to be taken up by the lung (epinephrine, norepinephrine, histamine, 5-HT) on both the amount and the distribution of vesicles. The purpose of this examination is to find out whether vesicular activity and pressure-independent transport can be either activated or inhibited by pharmacological means and how this correlates with data about permeability changes induced by these drugs.